U.S. patent number 8,196,421 [Application Number 12/308,015] was granted by the patent office on 2012-06-12 for system and method for controlled expansion valve adjustment.
This patent grant is currently assigned to Carrier Corporation. Invention is credited to Wayne P. Beagle, James W. Bush, Biswajit Mitra.
United States Patent |
8,196,421 |
Bush , et al. |
June 12, 2012 |
System and method for controlled expansion valve adjustment
Abstract
A method for controlling temperature pulldown of an enclosure
with a refrigeration system having a compressor, a heat rejecting
heat exchanger, an expansion valve, and an evaporator comprises
circulating a refrigerant through the refrigeration system, sensing
a parameter of the enclosure, determining a desired evaporator
pressure based upon the parameter sensed, and adjusting the
expansion valve as a function of the desired evaporator
pressure.
Inventors: |
Bush; James W. (Skaneateles,
NY), Beagle; Wayne P. (Chittenango, NY), Mitra;
Biswajit (Charlotte, NC) |
Assignee: |
Carrier Corporation
(Farmington, CT)
|
Family
ID: |
38778949 |
Appl.
No.: |
12/308,015 |
Filed: |
June 1, 2006 |
PCT
Filed: |
June 01, 2006 |
PCT No.: |
PCT/US2006/021124 |
371(c)(1),(2),(4) Date: |
December 04, 2008 |
PCT
Pub. No.: |
WO2007/139554 |
PCT
Pub. Date: |
December 06, 2007 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20090241566 A1 |
Oct 1, 2009 |
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Current U.S.
Class: |
62/224 |
Current CPC
Class: |
F25B
9/008 (20130101); F25B 49/02 (20130101); F25B
2700/21175 (20130101); F25B 2309/061 (20130101); F25B
2700/197 (20130101); F25B 2600/2513 (20130101); F25B
2700/2104 (20130101) |
Current International
Class: |
F25B
41/04 (20060101) |
Field of
Search: |
;62/114,222,224,225 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
|
0424474 |
|
Nov 1997 |
|
EP |
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1369648 |
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Dec 2003 |
|
EP |
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2001147048 |
|
May 2001 |
|
JP |
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Other References
Supplementary European Search Report and the European Opinion of
the International Searching Authority, or the Declaration;
PCT/US2006021124; Apr. 10, 2012. cited by other.
|
Primary Examiner: Norman; Marc
Attorney, Agent or Firm: Cantor Colburn LLP
Claims
The invention claimed is:
1. A method for controlling temperature pulldown of an enclosure
with a refrigeration system having a compressor, a heat rejecting
heat exchanger, an expansion valve, and an evaporator, the method
comprising: circulating a refrigerant through the refrigeration
system; sensing a parameter of the enclosure; determining a desired
evaporator pressure based upon the parameter sensed; and adjusting
the expansion valve as a function of the desired evaporator
pressure; wherein the step of adjusting the expansion valve as a
function of the desired evaporator pressure decreases an actual
evaporator pressure from a supercritical pressure to a subcritical
pressure.
2. The method of claim 1, wherein the parameter of the enclosure is
temperature.
3. The method of claim 1, and further comprising: determining an
actual evaporator pressure within the evaporator; and adjusting the
expansion valve as a function of the actual evaporator
pressure.
4. The method of claim 1, wherein the refrigerant operates in a
transcritical refrigeration cycle.
5. The method of claim 1, wherein the refrigerant is carbon
dioxide.
6. The method of claim 1, wherein the expansion valve is an
electronic expansion valve.
7. A refrigeration system for cooling an enclosure comprising: a
compressor for compressing a refrigerant to a gas cooler pressure,
wherein the gas cooler pressure is a supercritical pressure; a gas
cooler for cooling the refrigerant; an evaporator for heating the
refrigerant, wherein the evaporator has an evaporator pressure; an
expansion valve disposed between the gas cooler and the evaporator
and configured to reduce the pressure of the refrigerant from the
supercritical gas cooler pressure to a desired evaporator pressure,
wherein the desired evaporator pressure is a subcritical pressure,
the expansion valve decreasing an actual evaporator pressure from a
supercritical pressure to the subcritical pressure; and a sensor
for monitoring an enclosure parameter.
8. The refrigeration system of claim 7, wherein the refrigerant is
carbon dioxide.
9. The refrigeration system of claim 7, wherein the refrigerant
operates in a transcritical refrigeration cycle.
10. The refrigeration system of claim 7, wherein the enclosure
parameter is enclosure temperature.
11. The refrigeration system of claim 7, wherein the sensor is
configured to send a signal to a valve controller indicative of the
enclosure parameter.
12. The refrigeration system of claim 11, wherein the valve
controller is configured to adjust the evaporator pressure based
upon the enclosure parameter.
13. The refrigeration system of claim 7, wherein the expansion
valve is an electronic expansion valve.
14. A method for operating a refrigeration system having a
compressor, a heat rejecting heat exchanger, an expansion valve,
and an evaporator, the method comprising: circulating a refrigerant
through the refrigeration system; and adjusting an orifice of the
expansion valve as a function of a sensed parameter to decrease an
actual evaporator pressure from a supercritical pressure to a
subcritical pressure.
15. The method of claim 14, wherein the sensed parameter is
temperature.
16. The method of claim 14, wherein the sensed parameter is
pressure.
17. The method of claim 16, wherein the step of adjusting the
orifice of the expansion valve comprises sensing the evaporator
pressure and comparing the evaporator pressure to a desired
pressure.
18. The method of claim 14, wherein the refrigerant operates in a
transcritical refrigeration cycle.
19. The method of claim 14, wherein a valve controller receives the
sensed parameter and is configured to adjust the orifice of the
expansion valve based on the sensed parameter.
Description
BACKGROUND OF THE INVENTION
The present invention relates generally to refrigeration systems.
More particularly, the present invention relates to transcritical
refrigeration systems configured to improve temperature pulldown
after system start-up.
In a typical refrigeration system that utilizes a circulating
refrigerant, the refrigerant is circulated throughout a particular
refrigerated area to remove heat from that area. The refrigerant
enters the evaporator as a liquid or as a saturated mix of liquid
and vapor and the liquid is evaporated (i.e., it boils off to pure
vapor) as it absorbs heat from the refrigerated area. This process
takes place at a refrigerant temperature somewhat below the
temperature of the refrigerated area in order to facilitate heat
transfer from the area to the refrigerant. The flow of refrigerant
through the evaporator is normally regulated to maintain the
temperature of the vapor exiting the evaporator at some fixed
margin, or "superheat," above the saturated temperature of the
liquid-vapor mix. This assures that exactly enough refrigerant is
circulated to match the heat load of the refrigerated area. Because
the refrigerated area may not require constant cooling, the
refrigeration system may be turned off for a period of time,
thereby allowing the refrigerated area and the refrigerant to warm
to a temperature at or near the ambient temperature. When the
refrigerated area once again requires cooling, the refrigeration
system is turned on, and the refrigerant will initially go through
the process of evaporation at a temperature somewhat below the
ambient temperature. As the refrigerated area is cooled, the
temperature of the evaporating refrigerant will drop accordingly
until the refrigerated area reaches the desired temperature and the
system stabilizes again. The process of cooling a refrigerated area
from a warmer temperature following a system shutdown to a desired
cooler setpoint temperature is known as "pulldown."
Refrigerants containing chlorine have been phased out in most of
the world due to their ozone destroying potential.
Hydrofluorocarbons (HFCs) have been used as replacement
refrigerants, but these refrigerants also have high global warming
potential. "Natural" refrigerants, such as carbon dioxide, have
recently been proposed as replacement fluids. Unfortunately, there
are problems with the use of these natural refrigerants as well. In
particular, carbon dioxide has a low critical temperature, which
causes the evaporator temperature and pressure to be above the
critical point and in the supercritical region during start-up of
the refrigeration system. When the refrigerant is at a temperature
above the critical temperature, there are no separate liquid and
vapor phases and so the normal process of evaporation cannot take
place. When the evaporator temperature is supercritical there is no
such thing as "superheat," and therefore, the flow regulating
device is unable to operate properly. As a result, it becomes very
difficult to control the initial pulldown process that is necessary
to bring the refrigerated area to the desired setpoint temperature
and to return the refrigerant to a normal subcritical process.
Thus, there exists a need for a refrigeration system with improved
pulldown control when a transcritical refrigerant, such as carbon
dioxide, is used in a transcritical mode to provide cooling.
BRIEF SUMMARY OF THE INVENTION
The present invention is a system and method for controlling
temperature pulldown of a refrigerated enclosure with a
refrigeration system having a compressor, a heat rejecting heat
exchanger, an expansion valve, and an evaporator. The method
comprises circulating a refrigerant through the refrigeration
system, sensing a parameter of the enclosure, determining a desired
evaporator pressure based upon the parameter sensed, and adjusting
the expansion valve as a function of the desired evaporator
pressure.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates one embodiment of a refrigeration system
according to the present invention.
FIG. 2 is a process flow diagram illustrating the steps executed in
performing a temperature pulldown method according to the present
invention.
FIG. 3 illustrates a graph relating pressure to enthalpy for the
refrigeration system of FIG. 1 after system start-up and prior to
application of the temperature pulldown method.
FIG. 4 illustrates a graph relating pressure to enthalpy after
start-up and a first application of the temperature pulldown
method.
FIG. 5 illustrates a graph relating pressure to enthalpy after
start-up and a second application of the temperature pulldown
method.
FIG. 6 illustrates a graph relating pressure to enthalpy after
start-up and during steady-state operation after pulldown.
DETAILED DESCRIPTION
FIG. 1 illustrates a schematic diagram of refrigeration system 20,
which includes compressor 22, gas cooler 24, expansion valve 26,
evaporator 28, evaporator sensor 30, enclosure sensor 31, and valve
controller 32. Compressor 22 may comprise any type of compressor
including, but not limited to, reciprocating, scroll, screw, rotary
vane, standing vane, variable speed, hermetically sealed, and open
drive compressors.
Refrigeration system 20 is useful wherever a cooling source is
needed, such as in temperature control units for buildings and
automobiles. However, refrigeration system 20 will be described
generically in reference to an "enclosure" that requires cooling.
For example, the "enclosure" may be an office area in a building or
the food storage area in a refrigerated-type food transport
vehicle.
As shown in FIG. 1, refrigerant path 34 is formed by connection of
the various elements in refrigeration system 20. Refrigerant path
34 is created by a loop defined by the points 1, 2, 3, and 4. After
start-up of refrigeration system 20 from a non-operational mode to
an operational, cooling mode, refrigerant is first compressed
within compressor 22. The refrigerant then exits compressor 22 at
high pressure and enthalpy (point 2) and is directed through gas
cooler 24. The refrigerant loses heat in gas cooler 24, and exits
gas cooler 24 at low enthalpy and high pressure (point 3). Next,
the refrigerant exiting gas cooler 24 is throttled in expansion
valve 26. Expansion valve 26 is preferably an electronic expansion
valve (EXV). After going through an expansion process within
expansion valve 26 (point 4), the refrigerant is directed toward
evaporator 28. After being heated in evaporator 28 (point 1), the
refrigerant once again enters compressor 22, and the cycle
repeats.
As shown in FIG. 1, evaporator 28 of refrigeration system 20 is
disposed within enclosure 36, which represents an area that
requires cooling. A circulation element 38, such as a fan or
blower, is coupled to enclosure 36 and is configured to direct
streams of air 40 past evaporator 28 in an attempt to cool interior
42 of enclosure 36.
During initial start-up of refrigeration system 20, temperature T1
of evaporator 28 will be approximately equal to temperature T2 of
enclosure 36. In particular, if refrigeration system 20 has been in
the non-operational mode for an extended period of time, it is
likely that temperatures T1 and T2 are substantially equivalent to
the ambient air temperature outside enclosure 36. When using
standard, HFC refrigerants, the fact that temperature T1 of
evaporator 28 may be equal to the ambient temperature is not much
of a concern because HFC refrigerants typically have high critical
temperatures. As a result, refrigeration systems using HFC
refrigerants tend to run "subcritical." System operation and
cooling capacity are relatively easy to control in a subcritical
system due to the defined relationship between pressure and
temperature in the subcritical region.
On the other hand, when using transcritical refrigerants such as
carbon dioxide, the fact that temperature T1 of evaporator 28 may
be equal or close to the ambient temperature is problematic because
carbon dioxide has a relatively low critical temperature. The
critical temperature of carbon dioxide is about 87.8 degrees
Fahrenheit. In warm climates, it is common for the ambient air
temperature to exceed the critical temperature of carbon dioxide.
When this occurs, temperatures T1 and T2 may exceed the critical
temperature, thus resulting in a "supercritical" evaporator
temperature. As will be discussed in more detail to follow, in
order to achieve effective heat transfer between evaporator 28 and
enclosure 36 in such an environment, temperature T1 of evaporator
28 must be decreased to a subcritical temperature, i.e., a
temperature that is below the critical temperature of the
refrigerant. If temperature T1 remains supercritical during
operation of refrigeration system 20, the system will have minimal
cooling capacity and, as a result, it will be difficult or
impossible to pull down the temperature of enclosure 36 much below
the ambient temperature. This is especially detrimental when
refrigeration system 20 is used in, for example, a
refrigeration-type truck carrying perishable goods within enclosure
36. In that embodiment, it is critical that refrigeration system 20
is capable of pulling down temperature T2 of enclosure 36 to a low
temperature within a short amount of time so that the perishable
goods do not spoil. However, without having the capability to pull
down temperature T1 of evaporator 28 into the subcritical region,
refrigeration system 20 is almost useless as a cooling source. The
present invention provides a system and method for operating a
refrigeration system to pull down an enclosure temperature while
operating in either a subcritical or a supercritical cycle.
In refrigeration system 20, expansion valve 26, evaporator sensor
30, enclosure sensor 31, and valve controller 32 operate together
to enable sufficient enclosure temperature pulldown such that
refrigeration system 20 remains useful as a cooling source even
when operating in an environment wherein the ambient temperature is
above the critical temperature of the refrigerant. Evaporator
sensor 30 of refrigeration system 20 is coupled to evaporator 28,
and is configured to sense a parameter within evaporator 28 and
send a signal corresponding to the parameter to valve controller
32. Preferably, the parameter sensed by evaporator sensor 30 is
evaporator pressure, although other parameters (such as
temperature) that may be sensed and used to deduce pressure are
also contemplated. Similarly, enclosure sensor 31 of refrigeration
system 20 is coupled to enclosure 36, and is configured to sense a
parameter within enclosure 36, such as temperature, and send a
signal corresponding to the parameter to valve controller 32. Valve
controller 32 may use a combination of, for example, the evaporator
pressure, enclosure temperature, and the desired enclosure
temperature setpoint to determine a desired evaporator pressure
that will reduce the evaporator temperature to a subcritical
temperature and enable pulldown of the enclosure temperature to the
desired temperature setpoint.
In one embodiment of the present invention, enclosure sensor 31
includes a temperature transducer such as a thermocouple, RTD
(resistance temperature detector), or thermistor. Enclosure sensor
31 is configured to sense the temperature within interior 42 of
enclosure 36 and send a signal to valve controller 32. Based upon
the enclosure temperature, valve controller 32 determines the
proper adjustment to the evaporator pressure necessary in order to
attain the requisite heat transfer between evaporator 28 and
enclosure 36 and achieve the desired enclosure setpoint
temperature.
Furthermore, in one embodiment, expansion valve 26 is an electronic
expansion valve (EXV) and evaporator sensor 30 includes a pressure
transducer embedded in an evaporator tube to measure the
refrigerant pressure. The pressure transducer provides a feedback
signal to valve controller 32 which accordingly controls the
movement of expansion valve 26. The EXV includes is a mechanical
valve coupled to a stepper motor to control the opening and closing
of the valve orifice. The stepper motor responds to the valve
controller input by opening or closing the valve orifice as
necessary. Typically, the pressure drop is modified by controlling
the size of an orifice or flow restriction disposed within
expansion valve 26.
For normal steady-state operation where the evaporator is in a
subcritical state, evaporator sensor 30 may additionally include a
temperature transducer in order to determine superheat of the
refrigerant vapor exiting evaporator 28 by comparing the
temperature of the vapor to the saturated pressure within
evaporator 28.
FIG. 2 is a process flow diagram of a method 50 for controlling
temperature pulldown of an enclosure with a refrigeration system.
For purposes of example, method 50 will be discussed in reference
to refrigeration system 20 of FIG. 1.
Method 50 begins at step 52 by circulating a refrigerant through a
refrigeration system, such as refrigeration system 20. Method 50
continues at step 54 by sensing a parameter of an enclosure that
requires cooling. In one embodiment of the present invention, the
sensed parameter is the temperature of enclosure 36. Next, in step
56, a desired evaporator pressure is determined based upon the
sensed parameter within the enclosure. Any parameter or combination
of parameters that enables refrigeration system 20 to determine the
desired evaporator pressure is within the intended scope of the
present invention. Then, in step 58, the expansion valve is
adjusted as a function of the desired evaporator pressure. In one
embodiment, expansion valve 26 is adjusted to lower the evaporator
pressure from a supercritical pressure to a subcritical pressure.
After adjusting the expansion valve in step 58, an actual
evaporator pressure is determined in step 60, such as with
evaporator sensor 30. Finally, in step 62, the expansion valve is
adjusted as a function of the actual evaporator pressure determined
in step 60. It is important to note than in some instances, it may
be necessary to perform steps 54-62 continuously or at defined
intervals, as indicated by arrow 64, in order to achieve or
maintain the desired enclosure setpoint temperature.
In some instances, the various steps comprising method 50 may be
performed in a slightly different order. Furthermore, one or more
of the steps may be omitted without departing from the intended
scope of the present invention. For example, steps 60 and 62 may be
omitted such that method 50 adjusts the expansion valve based
solely on sensing the enclosure parameter and not on the actual
evaporator pressure as well.
By performing method 50, it is possible to pull down the enclosure
temperature in a refrigeration system that utilizes any type of
refrigerant, operating in either subcritical or transcritical
cycles. However, method 50 is particularly useful in conjunction
with refrigeration systems configured to operate in a transcritical
mode. As discussed previously, these types of systems typically run
supercritical when used in a hot ambient temperature. The system
and method of the present invention enables pulldown of the
enclosure temperature even in hot ambient conditions. Thus, the
present invention allows a refrigeration system to maintain the
evaporator in a subcritical state even when operating in an
environment above the critical temperature of the refrigerant being
used.
FIG. 3 illustrates a graph relating pressure to enthalpy after
start-up of refrigeration system 20 and prior to application of
temperature pulldown method 50. As shown in FIG. 3, refrigeration
system 20 is configured to circulate carbon dioxide. However, it
should be understood that carbon dioxide is used merely for
purposes of example and not for limitation. Furthermore, the cycle
in FIG. 3 assumes that the heat exchangers in refrigeration system
20 are ideal and that the pressure within evaporator 28 is held
substantially constant.
In FIG. 3, vapor dome V is formed by a saturated liquid line and a
saturated vapor line, and defines the state of the refrigerant at
various points along the refrigeration cycle. Underneath vapor dome
V, all states involve both liquid and vapor coexisting at the same
time. At the very top of vapor dome V is critical point P. The
critical point P is defined by the highest temperature and pressure
where saturated liquid and saturated vapor coexist. In general,
compressed liquids are located to the left of vapor dome V, while
superheated vapors are located to the right of vapor dome V. As
critical point P is approached, the properties of both liquid and
gas become the same. Thus, above the critical point, there is only
one phase. In particular, above its critical pressure, a substance
cannot be separated into liquid and vapor phases.
As shown in FIG. 3, within vapor dome V the temperature of the
refrigerant remains constant at a specified pressure. Thus, the
pressure and temperature of a refrigerant in the subcritical region
are directly related. However, outside of vapor dome V, there is no
specific relationship between temperature and pressure. For
example, the pressure within evaporator 28 (between points 4 and 1)
remains around 1200 psia, but the temperature within evaporator 28
increases from about 85 degrees Fahrenheit (point 4) at the inlet
of evaporator 28 to about 100 degrees Fahrenheit at the outlet
(point 1). Therefore, outside of the subcritical region of vapor
dome V, the relationship between temperature and pressure
disappears.
In FIG. 3, refrigerant path 34 is the loop defined by the points 1,
2, 3, and 4. The cycle begins in the main path at point 1, where
the refrigerant is a low pressure, high enthalpy supercritical
fluid prior to entering compressor 22. After compression within
compressor 22, the refrigerant exits compressor 22 at high pressure
and enthalpy, as shown by point 2. Then, as the refrigerant flows
through gas cooler 24, enthalpy decreases while pressure remains
constant, and the refrigerant exits as a cooler supercritical
fluid. After exiting gas cooler 24, the refrigerant is then
throttled in expansion valve 26, decreasing pressure as shown by
point 4. Finally, the refrigerant is directed through evaporator
28, where it exits as a higher enthalpy supercritical fluid as
shown by point 1. As shown in FIG. 3, points 1, 2, 3, and 4 of the
refrigeration cycle reside above the critical point P. When every
point of a refrigeration cycle is located above the critical point
for the refrigerant, the cycle is known as a "supercritical" cycle.
In this supercritical region, the liquid and gas phases are no
longer clearly distinguishable from each other, and the refrigerant
remains a supercritical fluid throughout the entire cycle.
In a refrigeration system, the specific cooling capacity, which is
the measure of total cooling capacity divided by refrigerant mass
flow, may typically be represented on a graph relating pressure to
enthalpy by the length of the evaporation line. As shown in FIG. 3,
the specific cooling capacity of refrigeration system 20 after
start-up is represented by the length of evaporation line L from
point 4 to point 1. The specific cooling capacity determines the
amount of heat transfer possible between a refrigeration system and
an area to be cooled. In particular, the location of point 1 along
evaporation line L is directly related to the temperature at point
1 which in turn is generally proportional to the temperature of the
area to be cooled. Note that with an increase in pressure, the
constant temperature lines near point 1 curve towards the left.
Therefore, for a given enclosure temperature, with an increase in
pressure, the maximum possible specific capacity decreases as point
1 slides left along the constant enclosure temperature isotherm.
Also, for a given enclosure temperature, an increase in pressure
causes the evaporator temperature to increase, thereby decreasing
the available temperature differential between the enclosure and
the evaporator, and decreasing the heat transfer between the
refrigerant and the enclosure. As a result, there is an adverse
effect on the specific capacity.
In FIG. 3, both the enclosure temperature E1 of enclosure 36 and
the ambient temperature A of the air outside enclosure 36 are about
100 degrees Fahrenheit. Furthermore, in this example, the desired
temperature setpoint D of enclosure 36 is approximately 30 degrees
Fahrenheit. Thus, in order to cool enclosure 36 to desired setpoint
D, refrigeration system 20 must have sufficient cooling capacity.
In particular, what drives the heat exchange between evaporator 28
and enclosure 36 is the temperature difference .DELTA.T1 between
evaporator 28 and enclosure 36. As shown in FIG. 3, temperature
difference .DELTA.T1 is about 15 degrees Fahrenheit at point 4 and
decreases rapidly to about 0 degrees Fahrenheit at point 1. Due to
the small temperature difference, the cooling capacity of the
system is also small. Therefore, it is very difficult to pull down
the temperature within enclosure 36 to desired temperature setpoint
D (especially in a short period of time) without adjusting
expansion valve 26, such as by method 50 as discussed above.
FIG. 4 illustrates a graph relating pressure to enthalpy after
start-up and a first application of temperature pulldown method 50.
As shown in FIG. 4, the adjustment of expansion valve 26 has caused
the pressure of evaporator 28 to drop below vapor dome V and into
the two-phase subcritical region of the vapor dome. In particular,
the evaporator pressure has dropped from about 1200 psia to about
700 psia, while the gas cooler pressure has remained constant at
about 1600 psia. After the first application of method 50, points 2
and 3 of the refrigeration cycle remain above vapor dome V, while
points 1 and 4 now reside below vapor dome V. Whenever the gas
cooler pressure is above the vapor dome and the evaporator pressure
is below the vapor dome, the refrigeration cycle is known as a
"transcritical" cycle.
Inside of vapor dome V, the evaporator temperature remains
constant. As a result, at a constant pressure, temperature
difference .DELTA.T2 also remains constant within this region.
Therefore, unlike temperature difference .DELTA.T1 of FIG. 3 which
continuously varied even at a constant pressure, temperature
difference .DELTA.T2 is both known and constant at all times within
vapor dome V. In particular, within vapor dome V, temperature and
pressure are directly related. Therefore, in this subcritical
region, the temperature of the refrigerant determines the pressure,
and vice versa. This fixed relationship allows precise control of
both evaporator temperature and pressure. Thus, a particular
evaporator temperature may be achieved by adjusting expansion valve
26 to the evaporator pressure that corresponds with that
temperature. In particular, method 50 allows refrigeration system
20 to constantly monitor and control the temperature difference
between evaporator 28 and enclosure 36, and in turn, the cooling
capacity of the system.
As stated above, adjusting the pressure drop caused by expansion
valve 26 such that the evaporator pressure is now within the
subcritical region results in an increased refrigeration capacity.
This increased capacity is represented by the length of the
evaporation line from point 4 to point 1. The main factor
contributing to the increased refrigeration capacity is the large
increase in the enthalpy at the evaporator exit temperature. As
shown in FIG. 4, the evaporator capacity has increased over the
supercritical cycle of FIG. 3 even though the evaporator outlet
temperature has remained the same. In addition, not only has the
refrigeration capacity increased, but the ability to control the
capacity has also improved by the transition from a supercritical
cycle where there is no defined relationship between temperature
and pressure to a transcritical cycle where the relationship
between temperature and pressure is known.
It should be noted that decreasing the evaporator pressure further
for a given enclosure temperature may not necessarily increase the
capacity further since the lower pressure also decreases the
density of the vapor returning to compressor 22 at point 1, and
thus decreases the total mass flow of the circulating refrigerant.
The optimal pressure in evaporator 28 will be a tradeoff between
the increased specific capacity, as seen by comparing the
pressure-enthalpy diagrams of FIGS. 3 and 4, and the lower total
mass flow resulting from the lower vapor density at point 1.
Therefore, valve controller 32 must be programmed to determine the
optimal pressure in evaporator 28 for a given enclosure temperature
in order to maximize the net cooling capacity of the resulting
refrigerant flow.
As shown in FIG. 4, enclosure temperature E2 is still substantially
equivalent to ambient air temperature A after a first application
of method 50. This results from the fact that the evaporator
pressure has just dropped down into the subcritical region, and
there has not been a sufficient amount of time for heat to transfer
from enclosure 36 to the refrigerant flowing through evaporator 28.
However, as will be seen in the following figures, the system and
method of the present invention will result in a decrease in the
enclosure temperature to the desired temperature setpoint D over
time.
FIG. 5 illustrates a graph relating pressure to enthalpy after a
second application of temperature pulldown method 50. As shown in
FIG. 5, the controlled adjustment of expansion valve 26 has caused
the pressure of evaporator 28 to drop to a lower pressure within
vapor dome V. In particular, the evaporator pressure has dropped to
about 550 psia, while the gas cooler pressure has remained constant
at about 1600 psia. After the second application of method 50, the
refrigeration cycle is still operating as a transcritical cycle.
However, the pressure difference between the high side gas cooler
pressure and the low side evaporator pressure has increased.
As shown in FIG. 5, enclosure temperature E3 has dropped from about
100 degrees Fahrenheit to about 60 degrees Fahrenheit. This
decrease in enclosure temperature is a direct result of the
controlled adjustment of expansion valve 26 according to
temperature pulldown method 50. Without adjusting expansion valve
26 to decrease the evaporator pressure into a region of two-phase
flow, it would not have been possible to achieve the decrease in
the enclosure temperature under the present conditions.
By performing temperature pulldown method 50, refrigeration system
20 has been able to pull down enclosure temperature E3 closer
toward desired temperature setpoint D, which is about 30 degrees
Fahrenheit. However, since desired temperature setpoint D of
enclosure 36 is lower than enclosure temperature E3 as shown in
FIG. 5, it may be necessary to decrease the evaporator pressure
even further to lower the evaporator temperature to enable
sufficient heat transfer. This may be accomplished by once again
performing temperature pulldown method 50, as will be represented
graphically in FIG. 6.
It is important to note that from a control point of view, when the
enclosure temperature is reasonably below the critical temperature
of the refrigerant, it may no longer be necessary to monitor the
enclosure temperature and evaporator temperature. A metering of
refrigerant based on the evaporator superheat may be sufficient to
control the system operation.
FIG. 6 illustrates a graph relating pressure to enthalpy for the
final, steady state operation of the system where enclosure
temperature E4 is substantially equivalent to desired temperature
setpoint D after application of pulldown method 50 over a
reasonably short period of time. In particular, the temperature of
enclosure 36 has been pulled down from ambient temperature A in
FIG. 3 to desired temperature setpoint D in FIG. 6 through
controlled adjustment of expansion valve 26. When an enclosure
temperature reaches and maintains a desired setpoint temperature,
the refrigeration system is said to be in steady state operation.
At steady state operation, it is no longer necessary to control
expansion valve 26 as described previously in order to maintain
enclosure temperature E4 at desired temperature setpoint D. At
steady state, refrigeration system 20 may continue to operate by
application of a method similar to method 50 described above.
However, refrigeration system 20 may alternatively use any number
of other devices and methods to control the evaporator temperature
during steady state operation of refrigeration system 20. For
example, refrigeration system 20 may include an additional sensor
disposed near the outlet of evaporator 28 that is configured to
sense the temperature of the refrigerant flowing through the outlet
and control temperature of the refrigerant within the evaporator
based upon this sensed value.
Although the present invention has been described in reference to
three applications of method 50 prior to reaching steady state
operation, embodiments that require more or less applications of
method 50 are within the intended scope of the present invention.
In particular, the number of applications required depends on many
factors, including the desired efficiency, the desired time to pull
down to the setpoint temperature, and the desired size of the
evaporator pressure changes to maintain effective performance
during pulldown. Therefore, the present invention has been
described in reference to three applications of temperature
pulldown method 50 for purposes of example and not for
limitation.
In addition, it should be understood that carbon dioxide was used
as the refrigerant for purposes of example only. The system and
method of the present invention may be used with any other type of
refrigerant without departing from the intended scope of the
present invention.
Although the present invention has been described with reference to
preferred embodiments, workers skilled in the art will recognize
that changes may be made in form and detail without departing from
the spirit and scope of the invention.
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